Between the waterproofing layer and roof deck there are layers of materials that can be installed. Some layers are mandated by the building code, some are mandated by the waterproofing layer manufacturer. Vapor barriers try to keep moisture from inside of the building diffusing and condensating into the roofing layers above the deck. Air barriers attempt to stop any gusts of air from pushing on the roofing system from the bottom and could be tied into the air pressure systems of the building. Insulation prevents hot and cold energy leaving or entering the building. Coverboards are dense materials that are used just under the waterproofing layer to protect the roofing system from traffic or next to the roof deck to protect the roofing system from fire.